In the field of aircraft manufacturing, it is already known to use the pressure differential between the air in the pressurized cabin and the ambient air surrounding an aircraft for generating or recovering energy. Namely, the cabin exhaust air that is removed or allowed to escape from the cabin can be used for generating or recovering energy due to its higher pressure relative to the ambient environment outside of the aircraft. For example, German Patent Laying-Open Publication 3,801,042 describes a system in which aircraft cabin exhaust air passes through an expansion turbine to drive the turbine and thereby drive a compressor. In that system, the expanded, cooled air exiting from the expansion turbine is used to cool a heat exchanger for the cabin air supply preparation plant, while the compressor compresses external ambient air to be delivered into the cabin.
It is also known, for example from European Patent Application 0,035,909, to provide bleed air or tap air from an engine bleed air discharge port of a jet engine, through a tap air duct, and into the air conditioning plant. Thus, the tap or bleed air is a source of fresh air and energy for driving the air conditioning plant. However, because the tap air bled from the jet engines has a high temperature, it is precooled, as is typical, before being delivered to the air conditioning plant. The precooling is realized by means of one or two heat exchangers. The hot tap air stream flows through one passage of the heat exchanger(s) and a cooling air stream flows through the other passage of the heat exchanger(s). The cooling air stream is a ram air stream and/or a cabin exhaust air stream, which respectively pass through the heat exchanger(s) and take up thermal energy given off by the tap air stream. The cooling air stream, which has now been heated, is simply discharged overboard to the outside environment around the aircraft through a discharge duct, without making any use of the thermal energy carried by the cooling air stream.